In recent years, a reduction in size and weight, an increase in power and an increase in efficiency have been required for permanent magnet motors used in hybrid vehicles, hard disk drives or the like. Such a permanent magnet motor has a configuration in which permanent magnets are buried in the outer periphery of a rotor and coils are arranged in the inner periphery of a stator, for example, as shown in Patent Document 1 (JP-A 2007-306735).
In realizing the reduction in size and weight, the increase in power and the increase in efficiency of the permanent magnet motor, further improvement of the magnetic characteristics is required for the permanent magnets to be buried in the permanent magnet motor. The permanent magnet includes ferrite magnets, Sm—Co-based magnets, Nd—Fe—B-based magnets, Sm2Fe17Nx-based magnets, etc.; and in particular, Nd—Fe—B-based magnets having high coercive force are used as the permanent magnets for permanent magnet motors.
Here, as a method for producing permanent magnets for use in permanent magnet motors, a powder sintering method is generally used. In the powder sintering method for use herein, a raw material is first pulverized with a jet mill (dry pulverization) to produce a magnet powder as shown in FIG. 9. Thereafter the magnet powder is placed in a mold, and press-molded to a desired shape while applying thereto a magnetic field from the outside. Then, the solid magnet powder thus molded to the desired shape is sintered at a predetermined temperature (for example, 1100° C. in the case of the Nd—Fe—B-based magnet), thereby producing the permanent magnet.
Recently, the demand for reducing the size and the weight of a permanent magnet motor is increasing; and in case where a permanent magnet motor is down-sized and in order to make the down-sized permanent magnet motor keep the same torque as before, the rotor must be rotated at a high speed. In high-speed rotation, eddy current is generated in the permanent magnets buried in the motor. When permanent magnets integrally molded according to the above-mentioned powder sintering method are used in such a permanent magnet motor, the eddy current shall be generated throughout all the permanent magnets.
When the eddy current is generated in the permanent magnets, the temperature of the permanent magnets rises. With the increase in the temperature thereof, the coercive force of permanent magnets lowers, and therefore the motor torque also lowers.
Patent Document 2 (JP-A 2001-25189) describes a permanent magnet as integrally so formed that a highly-insulating ferrite magnet is arranged between plural Nd magnets. Even when eddy current has occurred in the permanent magnet having the constitution, the eddy current could stay inside the individual Nd magnets as divided by the ferrite magnet, and therefore the eddy current may be reduced.
Patent Document 1: JP-A 2007-306735 (Pages 4 to 6, FIG. 1)
Patent Document 2: JP-A 2001-25189 (Pages 3 to 4, FIG. 2)